The present invention relates to a bellows-type gas meter comprising two compartments each having a diaphragm as well as a shaft common to said two compartments, said shaft having two eccentric cam assemblies one of said eccentric cam assemblies being connected with the crank assemblies of said diaphragms and the other one of said eccentric cam assemblies being connected with the crank assemblies of two valves creating in a cyclic mode passages between the four chambers into which said two compartments are divided by said diaphragms and a gas inlet or a gas outlet respectively, said two eccentric cam assemblies being displaced relative to each other by an angle so that each such valve during its valve closing cycle closes before the diaphragm operating in conjunction therewith moves into its final position at the end of each filling cycle.
The closing of each valve before the diaphragm operating in conjunction therewith moves into its final position, reduces the typical error of a bellows-type meter depicted in FIG. 1. Typically, the positive error of a bellows-type gas meter, meaning that the gas flow indicated by the meter is higher than the actual gas flow, is highest at approximately 20% of the maximum gas flow rate for which said meter is designed, while typically the negative error is highest at the minimum gas flow rate through said meter thereby having a negligible effect on meter readout. However, the negative error of a typical bellows-type gas meter also peaks near the maximum flow rate through said meter, said negative error having substantial implications for gas metering. The early valve closure proposed by the present invention shifts the curve depicted in FIG. 1 in the positive direction for high gas flow rates thereby making said curve approach the zero error line.
However, the time lag which may be provided between the closure of the valve and the movement of the diaphragm of the compartment served by said valve into its final position is limited, since the chamber being filled through said valve must be filled sufficiently.